Inhibitors will be designed and synthesized for human purine nucleoside phosphorylase (PNP) to be used with dideoxyinosine in conjunctive therapy for AIDS. Purine nucleoside phosphorylase is responsible for the inactivation of ddI through phosphorolytic cleavage of the glycosidic bond. Inactivation of PNP should lead to a significant increase in the serum half-life of ddI. Inhibitor design will be based on the three- dimensional structure of PNP which has been determined by X-ray diffraction techniques and refined at 2.75 A resolution. Initial modeling studies will be based on energy minimization using the computer program MacroModel. As the project proceeds, new computational methodology developed by Dr. Shalloway will be applied to the design of PNP inhibitors. New classes of PNP inhibitors will include transition state analogs and purine nucleoside analogs with novel heterocycles. In addition, new experimental and computational methodology will be developed within this subject. Density functional theory, which provides for a quantum mechanical treatment of large systems, will be used to calculate energies and electron densities for PNP complexes with substrates and inhibitors. The Corning Electronic Structure Code will be used to perform these calculations. We will also explore new approaches for improving the reliability of defining the accessible volume in the active site. These techniques will include both empirical approaches and theoretical approaches in collaboration with Dr. Shalloway. Finally, realizing the need for analyzing many PNP/inhibitor complexes, we will develop new techniques for rapid, efficient data collection using synchrotron radiation provided by CHESS and new detector technology under development within MacCHESS.